Use gtk-doc to build user's manual as well as reference

First two chapters. More to follow.

See previous commit.

Previously we were setting refcount of freed objects to the inert value, which
was harmful because it caused further destroy()s of the freed object to NOT
call free() and hence hide the bug.  Indeed, after eb0bf3ae test-object
was double-free'ing objects and this was never caught on Linux.  It only was
caught as crashing on Mac.

Now we poison refcount upon freeing and check that it's valid whenever reading
it.  Makes test-object fail now.

See thread "Issue with cursive attachment" started by Khaled.
Turned out fixing this wasn't as bad as I had assumed.  I like the
new code better; we now have a theoretical model of cursive
connections that is easier to reason about.

Fails now.  Fix coming.  See thread "Issue with cursive attachment"
started by Khaled.  Test fonts were made by modifying test font
from Khaled to add more anchors.

In anticipation for upcoming fix for bug reported by
Khaled in thread "Issue with cursive attachment".

New API: HB_BUFFER_SERIALIZE_FLAG_GLYPH_EXTENTS

hb-shape now accepts --show-extents.

Patch from Simon Cozens.

This brings ot-fonts into almost-complete shape and mostly in par with
ft font.

Shouldn't cause *any* functional changes, but is more correct.

Ouch!

By default shape at upem (or ppem), and scale results.
Similar to work done in CoreText backend, but using upem as default.

To be turned into a useful HB_DEBUG_SHAPE infrastructure...

Ie, don't call get_positions() if positions are not
requested for serialization.

Needed for upcoming debug output changes.

See comments.

http://crbug.com/419769

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=64501

Fixes https://github.com/behdad/harfbuzz/pull/124

Fixes tests/cluster.

Fixes https://github.com/behdad/harfbuzz/pull/123

For https://github.com/behdad/harfbuzz/pull/123
Currently fails.  Fix coming.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=91559

fix static linking with icu-uc